118

CHAPTER- 5

Iron pyrite thin films

5.1 Introduction

5.2 Properties of iron pyrite

5.3 Iron pyrite based solar cells

5.3.1 Photo electrochemical solar cell

5.3.2 Pyrite/metals Schottky barrier

5.3.3 Thin layer solar cell

5.4 Methods of preparation of iron pyrite

5.5 Preparation of iron pyrite thin film using CBD technique

5.5.1 Stage: 1

5.5.2 Stage:2

5.5.3 Stage:3

5.5.4 Stage:4

5.5.5 Stage:5

5.6 Conclusion

Reference

5.1 Introduction

High production cost of solar cell materials is the major factor that limits the

commercial applications of photovoltaic cells. In such a context, much attention has

been given on highly absorbing photoactive semiconductors that are abundant and

nontoxic. Today the widely used material is crystalline silicon. This material poses no

direct problems for the environment, but has the disadvantage of high material

consumption and high-tech material processing. It is said that for 1 GW photovoltaic

instillation, approximately 8000 tons of high quality silicon is required. The other

leading materials of this field viz. GaAs, CdTe or CulnfSe, have the disadvantage of

containing toxic substances such as Cd, Te, Ga, In or Se which makes them less

attractive for mass production.

Iron pyrite is an eco-friendly material that is abundant in nature. It is a

promising candidate with a minority carrier diffusion length much larger than its

absorption length (200 A at 2 eV) [1] and higher carrier mobility even in thin film

form (200-300 cm2Ns) [2]. In the visible range this material has a high absorption

coefficient (a.) of Sx lOi cm' [3-5]. Table (5.1) gives the comparison of this material

with the other known absorbers. Fig.(5.1) shows the absorption length (1/a.) of FeS 2

in comparison with other semiconductor materials [6]. With such a high absorption

coefficient this material is projected as the most suitable material for sensitisation

type solar cells [7].

C-Si a-si :H CulrrSe, Culnfs, FeS 2

Energy gap 1.1 1.5-1.8 0.95 1.53 0.95(eV)Absorptionco-efficient 5 x 103 3 X 104 2 x 105 2 x 105 6 x 105

a (ern)" at2eVAbsorptionlength o' 2 x 104 3.3 X 103 ·5 X 102 5 X 102 1.7 X 102

(A)Quantumefficiency 25 - 25-30 20 80-90%'

Table (5.1) Comparative data ofFeS2 related to few well knownabsorber materials

5.2 Properties of iron pyrite

Unless specially mentioned, the term 'pyrite' refers to iron pyrite (FeS 2) , even

though the term actually refers to a particular structure. FeS 2 generally crystallises in

a pyrite structure, which can be visualised by replacing the Na atoms in the NaCl

structure with Fe atoms and Cl atoms by sulfur dimmers. It. also crystallises in

marcasite structure, an orthorhombic modification present in nature. Marcasite phase

often appears during preparation of pyrite, but it can be converted to pyrite by

annealing the sample under sulfur atmosphere at high temperature. Fig.(5.2) shows

the phase relation ofFe-S system [6].

Optical band edge reported for this material has a very wide range [5,8,9]. I. J.

Ferrer et al. [5] has summarised the values in the form of a table, Table (5.2). This

unusual behaviour is due to the fact that the general expression to determine the band

gap nature and the value of'E, (given below) is not applicable for this semiconductor.

(uhv)" = A (hv-Ej),

Eo being the transition energy (Eo is E, for direct transitions, Eo = Eg ± E, for indirect

transitions, where E, is the energy of the associated phonon) and n depending on the

transition type, n = ~, ~,2 or ~ for allowed indirect, forbidden indirect, allowed

direct and forbidden direct transitions respectively. This equation is defined to

describe electronic transitions in semiconductors between parabolic bands [10-12].

But this is not the case with pyrite. From band structure calculations carried out by

Bullett [13], it is known that the top of the conduction band is rather flat. Hence as the

pyrite band edges are different from those of ordinary semiconductors, it might be

necessary to modify the simple equations used to investigate direct or indirect

transitions.

tl...&...A.

5

411.,r'H1. Il(~

3

2.0JJ.l11 2.0J,1m2

1.5~m

1.0 J.1I111 ~

05 O.02J,1m~O.3

FeS ,,",uln8e 2 CulnS ICdTe GaAs .-Sl c-Sl

Fig.(5.l) Comparison of the absorption length for different semiconductormaterials.

10~i

Atomic Percenl SuI fur60 eo "10, , ,40,3fJ

i20

I '10I

o1800 ~~~.............--'1r'J"'W"W'''''''''''''''''''''''''''''''''''''~~--P1~",-",~"""",---"""",,,,,,,,,,,,,,,,,,1'''''I''''9'''

lG38·C

•••.eo-cS.P.

100

S

L2-tIIII,III ..

t-:-:-.,.----------------~!'i,.

••II,

\~--------- r------------------------------is,·- lIS.22"C

.0 SO GO 70

Weight Per-cent, Sultur302010

(arc)

ooFe

200

~ 1000-J----...........--------,."I........-ot

~ Il~C~-----...;. ........_----t:s.-I

~ Jg~c- ._._._._._._._._~._._._._._._._._._.-

QJ0.

E" 000re

400

Fig (5.2) Phase relations in the Fe-S system

121

Ea Transition type Experimental Sample nature(eV) Technique0.9 photocurrent natural and synth. single crystal

0.95 indirect quantum efficiency natural and synth. single crystal

0.9 reflectance natural and synth. single crystal

0.95 reflectance natural single crystal

0.96 allowed optical absorption natural single crystalindirect

1.6 indirect reflectance natural single crystal

0.9 direct reflectance synthetic samples

0.84 indirect quantum efficiency synthetic samples1.03 direct

0.9 quantum efficiency single and polycrystal0.84

0.95 indirect quantum efficiency thin films

2.62 direct transmittance thin films1.45 indirect1.05

1.95 forb. direct transmittance thin films1.12 forb. indirect

1.82 indirect absorption coefficient thin films

1.5 direct absorption coefficient thin films0.6 indirect

Table(5.2) Reported values ofEg and corresponding transition type of pyrite (FeS2)

In a chemical vapour transport (CVT) sample, typical Hall mobility of

electrons was in the range of 230-366 cm 2N s and carrier concentration in the order of

l016cm-3. A very high electron mobility of 1000 crrr/Vs was observed at a low

temperature of 150 K. Though n-type conduction is usually reported [14-15], there are

122

also reports of p-type conduction [16-19] in pyrite. The electrical resistivity of this

material is reported as a few Qcm [20].

Pyrite when acting as an electrode in electrochemical cell [6] reacts with water

to produce Fe3+, S04 2-, H+ and electrons according to the reaction

FeS2 + 8 H20 ~ Fe3+ + 2 S042- + 16 H+ + 15 e-

Under illumination and in the absence of electron donors such as Br' or 1-, the material

photo corrodes via. holes. The reaction is

FeS2 + 8 H20 + 15 h'~ Fe3+ + 2 S04- + 16 H+

Inthe presence of electron donor species, the system will have kinetic stability. Pyrite

has only a very narrow stability domain. This is obvious from Fig.(5.3) which

represents the potential-pH diagram for iron-sulfur-water system. Solid lines enclose

the areas of stability for the solid phase and the dashed lines define the equilibrium

between solution species.

Electrochemical treatment of atomic hydrogen based on the proton reduction

was found to be effective in increasing the photo effect. Hydrogen interaction with

both the pyrite surface and the bulk plays a significant role. Electrochemical treatment

is an etching process by which impurities such as FeS can be eliminated (by H2S

formation) or oxygen can be removed from the surface. In addition, neutralization of

bulkdefects by insertion of hydrogen also occurs.

5.3 Iron pyrite based solar cells

Pyrite is comparatively a new material in the field of photovoltaics and for the

same reason cells based on this are still on trial basis. Few of the reports that have

appeared on this material in this field are summarised below.

5.3.1 Photo electrochemical solar cells

Generally in photo electrochemical solar cells (PEe) based on semiconductors

like Si, GaAs, etc., there are severe thermodynamic restrictions for stability. However

in FeS2 these restrictions are minimized to a great extend due to the fact that photo

excitation in this material involves a non-bonding electronic transition that avoids

breaking of bonds.

A PEC based on n-type FeS 2 with high quantum efficiency and high stability

in presence of 1-/13- redox couple has been reported [20-22]. Fig.(5.4) shows the

photocurrent action spectrum across an n-FeS2/electrolyte (5M KI) interface. The

measured quantum efficiency at the maximum of spectral sensitivity exceeds 90%.

This indicates the possibility of using this material for solar energy conversion.

Figure (5.5) shows the output characteristics of a similar cell at 100 mWcm-2

illuminations. The electrolyte used was 4 M HI, 0.05 M 12 along with 2 M Ca12• This

cell reported a conversion efficiency of 2.8% [6].

A correlation between photovoltage and SlFe ratio is shown in Table (5.3).

Here Iph and Id are photocurrent and dark current respectively. The best results were

obtained for a S/Fe ratio close to 2 (in these cases Zn was used as the dopant).

S:Fe Zn Cone, (ug/g) Voc (mV) Iph/ld

1.98 2300 208 74

1.96 2000 193 165

1.94 3400 176 70

1.88 4500 160 15

Table (5.3) Correlation between photo voltage and SlFe ratio.

In all these cells the limiting factor for high efficiency is the high dark current,

which leads to a small photovoltage.

5.3.2 n- pyrite/ metal Schottky barriers

Different n-FeSymetal schottky barriers (metals like Pt, Au or Nb) were

fabricated by depositing transparent metal films (50-120 A0) on top of an

electrochemically etched pyrite surface [6-23]. This was performed in vacuum by

electron beam evaporation or chemical vapour transport. A schematic cross section of

a pyrite diode prepared in this manner is given in Fig.(5.6).

In spite of considerable reverse current (15 mNcm2 at 0.7V) and poor

rectifying behaviour at room temperature (j fOlWard/j reverse = 25 at 0.7 V), the n-FeS2/Pt­

barrier shows high short circuit photocurrents reaching values of 30 m/vcrrr' and

saturation photo currents of 100 rn/vcm' under illumination of 200 mW/cm2,

Fig.(5.7). The spectral response of this diode showed quantum yields of 40% and

••IFff(Jil.j FelOHl;

1.0 .~

o.n -> 0.6-..LLJX; 04tI'l

'" 0.2>

~

i 0..o0... -Q2~ Fe2

+

~ -0.4u.,

G:; -Q6 1------.....

-0.8 Fe

-1.0

o 2 4 6 8 10 12 14pH

Fig.(5.3) Potential- pH diagram for the iron-sui fur-water system at 25°C

3.83.2

\.'"

2.82 2.4

Energy I.V1.61.2

80

100 .....----------------.------...

20

>­u.i 60

e"EE 40•~c

Fig.(5.4) Quantum yield and spectral dependence ofphotoelectrochemical cell using iodine/iodide electrolyte

50-.----- __1OO",W Icm aVoc • "7 IIlV

Ff.O.1

. '_ • 42 mA I eM I

EJ'telenc~ • 2.8%

El

10 -

o

oI

o.~

.;.. I

0.2 0.3Photoyolc.ge I Y

I0.4 0.5

Fig.(5.5) Power output of a photo electro chemical cell usingiodine/iodide electrolyte

2000 A50-120 A

0.1 mm

0.1 mm

10mm

Fig.(5.6) A schematic cross section of a pyrite/metal diode

124

70%, at 1.3 eV photon energy under short -circuit and bias condition (0.7 V)

respectively. A high density of bulk defects (viz. sulfur deficiency) and surface states

were the factor that limited the junction performance [24].

5.3.3 Thin layer solar cell

Improvements in FeS 2 films have lead to development of thin film based solar

cells with a p-i-n structure [6]. Here an ultra thin film. (10-15nm) of pyrite is

deposited on large band gap materials like Ti02 WO)' ZnO, etc. so that the visible and

near infrared light is absorbed in the pyrite film. The strategy adopted is to develop a

solar cell where the absorber layer of FeS 2 absorbs the visible light and injects the

electrons into the conduction band of large band gap material. Thus generating a

photovoltage. The reaction of holes occurs with an electron donating electrolyte. Or in

other words, an appropriate redox electrolyte reacts with the holes like a p-type

contact. Such a p-i-n device, where a very thin layer of intrinsic pyrite is sandwiched

between suitable p and n-type window material, is capable of converting light into

electrical energy. Fig.(5.8) shows such a Ti02/FeS2/electrolyte interface. In contrast to

a conventional p-n junction where the device absorbs light as well as transport the

charge carrier, the pyrite-sensitised device separates the function of light absorption

and carrier transport. This type of cell was tried by Ennaoui et al.[7] and Fig.(5.9)]

shows its reported photoresponse. In that study it was found that FeS2 layers thicker

than 150Ao decreases the photo effect drastically.

5.4 Methods of preparation of iron pyrite

The varIOUS methods of preparation of FeS2 as reported In literature are

summarized in the form of a table, with mention to references.

150-(\11

ECJ 1004~..-.. 50.~VIC 0t»

-0

iCe -50~

(.)

-100-0.3 o 0.3

Potential I V

0.6

Fig.(5.7) Current-voltage characteristics of a pyrite/Pt diode

Fig.(5.8) Schematic morphology of the Ti02/FeS2

contactshowing electron injection

• 1H F.(CH~·"·

01" KI0.01111z

..

u

t4 16 U 2.0 Z.2 2J. 2.6 2.' 3.0

Fig.(5.9) Photo voltage vs. time for an electrode in thepresence of an iodine/iodide electrolyte

Method of preparation Properties/S tudi es/O bservations

A. Sulfurisation ofiron film [17,25-30] Resistivity - 10-20 cm;

Ref.[17], thermal evaporation of Iron p-type conduction;

powder on glass substrate at 200°C; band gap of 1.45 eY (direct) and 1.31 eY

sulfurisation under nitrogen flux for 30 indirect.

min.; Fe film kept at 280°C and sulfur

source at 180°C.

B. Sputtering [18,31-32] Conductivity - 40-1cm-1;

Ref.[18], magnetron sputtering; FeS2 target Carrier density -5xI018ern";

of 99.9% purity; .sputtering gas of sulfur Mobility - 5 cm'V'ls'

plasma and small quantities of Ar. p- type conduction

Sulfur content - 60 to 65%

c. Vapour phase epitaxial growth (VPE) MPSM (Microwave photoconductivity

[33] scanning microscope) studies.

Substrate-natural pyrite at 595°C; transport

agent-bromine; source-mixture of FeS2and

ZnS powder; source temperature 590°C;

duration of deposition-I week.

D. Flash evaporation [30,34] Annealing in an inert atmospheric leads to

Ref.[34], source- natural pyrite powder of loss of sulfur; Fe1_xS phase transfers to FeS 2

size 50-75 urn; pressure-l O'Torr; source by annealing in sulfur atmosphere; sulfur

temperature-I 400°C; substrate-glass or pressure-500 tOIT, time of sulfurisation-I20

Sn02 coated glass; substrate temperature mm., temperature 350°C; as-prepared

-120°C. resistivity-2x 10-20cm sulfurised resistivity-

2xIO-1Qcm; optical absorption-Zx l O' cm'

at hv > 1.5 eVe

126

E. MOCVD [7,35-37] For substrate temperature greater than

Ref[35], deposition of iron by 170°C, conductivity greater than 103

decomposing iron pentacarbonyl onto a hot (Qcm)-I; TRMC (Time Resolved

substrate, In the presence of hydrogen Microwave Conductivity) measurements.

sulfide gas; source temperature 25°C;

reaction pressure 1000 mbar; growth

temperature"J 140°C.

F. Electrodeposition [38]

Direct electrodeposition of Fe and S in an

aqueous solution of FeS04 and Na2S203 on

titanium sheets for 60 min; pH adjusted

using H2S04; post annealing In sulfur

atmosphere in 40 min. around 500° in

nitrogen atmosphere.

EPMA analysis showed Fe/S ratio to be

around I; XPS analysis revealed the

formation of FeS; stoichiometric poly

crystalline pyrite thin films were obtained

after post annealing in sulfur atmosphere.

G. CVT (Chemical Vapour Transport) Absorption co-efficient at 632nm-6.4xIOs

[3,39] ern"; Eg-0.95 eV (indirect); complex

Ref.[3], FeS2 powder prepared by ampule refractive index n = 4.032-3.245i at 632.8

synthesis; 2 g FeS2 powder and 5 mg As nm. It was found that light is absorbed

dopant taken in quartz ampule: pressure within a layer of 160Ao ofFeS2•

10-s mbar along with 0.5 mg/crrr' bromine;

FeS2 powder heated to 800°C and the

powder free end to 550°C for 10 days, the

next 10 days the temperature gradient is

reversed to get pyrite.

H. Spray pyrolysis [40-42] Resistivity- 0.16 Qcm; presence of

Ref.[40], spray solution-0.03 M, x ml FeCl3 impurity phase of Fe1_xS; band gap 0.82 eV

and 2.24 x ml thiourea; carrIer gas (indirect), absorption coefficient-I.6x1OS

nitrogen; temperature of substrate-Sotf'C, cm' at 850 nm; doping with ruthenium

vacuum- 28 inch Hg; duration of spray-30 found to be good; photoconductivity

min; evaporation of free sulfur was also studies.

done to reduce the oxygen partial pressure.

127

I. CBD (Chemical Batl, Deposition) [43] X-ray fluorescence study reveled Fe-

Ref.[43], prepared in organic medium; 0.02 33.45%, S-66.55%; TRMC, ICP-MS and

M iron pentacarbonyl mixed with 0.04 M SEM studies; in a photo electrochemical

suifur dissolved in 250 ml xylene under setup with an electrolyte of HI, Cal, and 12,

argon atmosphere; temperature of they showed good photoelectric behaviour

deposition-140°C; time of deposition-few with high quantum efficiency.

minutes.

J. Sulfurisation ofiron oxide film [44] p-type conduction; band gap-I.3eV

Ref.[44], reaction of Fe304 or Fe203 with (direct), 0.93 eV (indirect); conversion of

elemental sulfur vapour in open or closed Fe304 to pyrite found easier.

ampoules; temperature of sulfurisation-

350°C; duration of sulfurisation 0.5-2hrs.

5.5 Preparation of iron pyrite thin film using CBD technique

In the present work preparation of iron pyrite (FeS 2) thin films was tried using

chemical bath deposition technique in aqueous medium. This chapter summarises the

various attempts made to prepare stoichiometric pyrite films. The complete work

done on this material is presented, highlighting various stage of development.

Glass slides were first washed in soap solution and then in freshly prepared

chromic acid solution, so as to remove the oil particles. It was then washed in running

water and later ultrasonic cleaning was done. Once again it was washed in double

distilled water and dried in an oven. This formed the substrate for thin film

deposition. On few instances Sn02 coated glass substrates were also tried and the

details regarding preparation of Sn02 layer is already included in section (2.2.2).

5.5.1 Stage:1

Ferric chloride (FeCI2) was chosen as the Fe source and thiourea [CS(NH2)2]

as the sulfur source. In order to control the rate of reaction between these two

reagents, it was necessary to complex the Fe ions. Triethanol ammine (TEA) was

128

selected as the suitable complexing agent. The complex was then dissolved in

ammonium hydroxide solution before addition of thiourea under constant mechanical

stirring. Cleaned glass substrates were introduced into the beaker in vertical position.

The reaction mixture was maintained at a constant high temperature. The expected

reaction was

2[Fe(TEA)]3++ 4 CS(NH2)2 + 8 OH-~ 2 FeS2+ 2 TEA + 4 CO(NH2)2 + 4 H20

After trying a large number of permutations and combinations of deposition

parameters like concentration of reactants, temperature of deposition, duration of

deposition, etc., the final procedure adopted was as follows: To 10 ml of I M FeCl3

solution, 5 ml of TEA was added and stirred well. 20 ml of NH40H solution was

added to this in order to dissolve the iron complex. Later 10 ml of IM Thiourea was

mixed with stirring. The duration of deposition was about 80 minutes at 70°C. After

deposition the samples were washed in water and dried in hot air. The resulting film

had a brown colour. Thickness measured by gravimetric method was around 0.4 urn

for single dip.

Optical absorption studies conducted on these samples showed a direct band

gap of 3.1 eV and indirect band gap of 1.84 eV. Though iron pyrite has a very wide

range of reported band gaps as shown in Table (5.2), these values were found to be

higher than all the reported values. The resistivity of this material was found to be

higher than the expected value by a few orders of magnitude (RY'"'JI07Q cm). XRD

analysis done on this thin film revealed the amorphous nature. These samples were

annealed at various temperatures (lOO-350°C) in air as well as in vacuum, inorder to

improve the crystallinity. But this yielded no results, except for the fact that the films

got completely cracked at high temperature. Fig.(5.10) shows the SEM micrograph of

such a sample under IK magnification when annealed at 350°C in vacuum.

The elemental analysis of this as-prepared phase was also done. Figure (5.11)

shows the XPS depth profile done using X-rays from Al anode operated at 3 kV. The

bottom layer of the figure corresponds to the surface layer of the film while the top

region refers to the Si02/glass substrate. The X-axis gives the binding energy in eV,

while each horizontal line corresponds to the signal collected after each etching cycle.

The binding energies of2p 3/2 of Fe at 710 eV, 2PY:I of Fe at 72.5 eV and Is of 0 at 530

eV, along with the absence of any suIfur signal reveals the formation iron oxide [45]

and not iron sulfide. Binding energy of surface oxygen at 532 eV corresponds to

Fig (5.10) SEM micrograph of as-prepared film annealed at

350°C in vacuum

~..Q)QC~-Z

:...."..,.

'-'

.....)..of~

....-4.....:.......,...

\eJ

Q)

S-4~~.......Q)

~ s-.> "".......Q) I"-"" u:

~

a;) RJ .Jn S... --~-- -t~

~

o:,...0) ......

><~

...-.............~~

........."

ec.....~

physisorbed oxygen [46]. Figure (5.12) shows the atomic concentration of this depth

profile. It is obvious that this oxide phase of iron contains iron double than of oxygen.

The as-prepared films from 1 M FeCI) and IM Thiourea were thus found to be

uniform film of iron oxide rather than iron sulfide.

5.5.2 Stage: 2

As a next stage of trial, the relative concentration of iron in the CBD reaction

mixture was reduced to half. Here 10 ml of 0.5 M FeCI) was made to react with 10 ml

of IM Thiourea. 3 ml and 5 ml of TEA and NH40H respectively were used for

controlling this reaction. Here the time of deposition was 1 hr. at 70°C. The as­

prepared films had a yellowish brown colour, unlike the earlier case. The thickness of

these films was very less and gravimetric measurements proved this to be around 0.2

urn for first dip, 0.4 urn for second dip and 0.5 urn for third dip. The optical

absorption studies- showed a slight shift of band gap towards the earlier reported

values. The plot of (uthv)" versus hv plot revealed an indirect gap of 2.94 eV while

the plot of (athv) 1/2 versus hv plot revealed an indirect gap of 1.67 eV (a stands for

absorption coefficient, t the thickness of the film and hv the incident photon energy).

This is shown in Fig.(5.13) and Fig.(5.14). From XRD analysis Stage:2 samples were

also found to be amorphous.

As-prepared fresh samples were then analysed by XPS, after removing the

surface layer by argon ion sputtering for 2 minutes. Strong signals of iron and sulfur

with binding energies around 707 eV and 161 eV respectively was observed as in

Fig.(5.15). This analysis of fresh sample was done at RSIC, Madras. Though the exact

percentage composition of the material could not be evaluated, the stoichiometry was

found to be poor. The same sample was analysed in Japan, after a period of two

months. The XPS depth profile in Fig.(5.16) revealed interesting results. It was

observed that sulfur signal was present only on the bottom layers. Iron sulfide was

found to get converted to oxides of iron from the top surface towards the depth of the

film. In the converted region Fe signal showed a B.E of 709.3 eV corresponding to

iron oxide. In the inner layers of the film, Fe signal showed a slight kink 706.7 eV

corresponding to FeS 2• The corresponding sulfur peak was at 160.4 eVe

The above results can be justified as follows: As the film preparation is carried

out in an aqueous alkaline medium, there is chance of formation of small quantities of

01

01

01

Fel

Fel

Fel

---------~--------

Fel

01

I--

l,

I.

,--

,.

20

10

0

90~-

80 70

tIP6

0. u

50. .re

40 30.L~-·'·-·~,,-··

Sn

lS

nl

.Q2

"~..

'I1a

ll.

SO

l..

.-i1

1...

__

_.

...C

l.

----~-Ini

__~-~

1_,_~

_------e'---,-~§ll~.'_'l

J_S

~__.:~~11~~~___

Si~

__--~

---'--

·-"-"'I~:=::~·,:;~~·;5';~

dk.--""~----~l

_.-..

..~

-==

-_+

_:::~-.::.:

--.~:-':'..~~i-'=":·~:;:;":"'f:=·--=:···~t~-:·~·~·::'-::::-=·":..:~

10 o

o1

02

03

040

50

60S

pu

tte

rT

im8

(min

)Fi

g..(

5.12

)P

erce

ntag

eco

mpo

siti

ono

fas

-pre

pare

dfi

lmo

fst

age:

1

70

.-+~-_.

80

4.0

1.5

1.0

Indi

rect

Eg

-1.

67eV

0.0

I(

I

0.0

0.5

1.0

3.5

'i

3.0

2.5

0.5

1.5

Cl) ....., '§ .0 a

2.0

S ...-,

.> ...d tj '-'"

2.5

2.0

1.5

o~...;-

,,

!,i

1.0

4

Indi

rect

Eg

=2.

94eV

Cl) ....., ·a ~ .e cd

6N

...-,

.

> ..c: E '-'"

810•

i

2

hv(e

V)

Fig.

(5.1

3)(a

thy)

2vs

.hv

plot

ofa

s-pr

epar

edfi

lmo

fsta

ge:2

hv(e

V)

Fig.

(5.1

4)(a

thy)

1/2

vs.

hvpl

oto

fas-

prep

ared

film

ofs

tage

:2

2600

~

~

o§ 2400

.c"""~-.....--Cor;;CG)

C 2200~

2000

Fe 2p

6500

.,--...

o~t:~

.0"""~'-"

co~

cE6000......

690 700 710 720Binding Energy (eV)

S 2p

730

155 160 165 170

Binding Energy (eV)175

Fig.(5.15) XPS signals of stage : 2 thin films

oorf

tI2

~0e

Nf+.40

"00.t:0~

~....

~~

N0'-"'" OD

~StI2

f+.4~ 0

~0~

~ ~~ ~

tI2

~ 0

~.s0~

~ t.r=e~

.;ie,0

"0Cl)

~".......\0...-4

V")'-"'"ob.....~

iron oxide impurity phase. This is possible as pyrite has a very narrow stability

domain as shown in Fig.(5.3) [6]. This metal oxide impurity imparts hydrophilicity to

the pyrite film [46]. This lead to the slow corrosion of the already sulfur deficient

phase of iron sulfide into oxides of iron. This hydrophilicity is obvious in the as

prepared film, in the form of reduction of resistance by two orders of magnitude,

when the film is kept in humid atmosphere.

Thus in Stage:2, with higher relative concentration of sulfur in the reaction

mixture, it was possible to incorporate small quantities of sulfur to the as- deposited

films. But disproportionate increase in sulfur resulted in no filrri formation. Stage:2

could only lead to a sulfur deficient pyrite phase which was found to be unstable in

ambient conditions.

5.5.3 Stage: 3

Trials were done to substitute thiourea with more active sulfur releasing

reagents like sodium sulfite and thioacetamide. Sodium sulfite was found to be highly

reactive with iron and hence it was difficult to control the precipitation to get film

formation.

With thioacetamide the reaction could be controlled after many trials. 10 ml of

0.25 M FeCl3 was mixed with 2 ml TEA and 5 ml NH40H solution and stirred well.

To this mixture 20 ml of 0.5 M thioacetamide was added. The reaction was complete

within 20 minutes at a temperature of 70°C. For considerable thickness, two or three

successive dipping was required.

This sample when analysed by XPS (7 months after preparation), showed

strong signals of iron and sulfur [Fig.(5.17)]. The 3p signal of Fe at 54.2 eV and the

2p3/2 signal of S at 161.7 eV supported the formation of iron sulfide [47]. But the

atomic concentration (%) graph as shown in Fig.(5.18) revealed another major draw

back. Here though 10% of sulfur was present on the surface, the concentration of

oxygen through out the sample was found to be higher than the concentration of Fe.

In all the other cases though oxygen was present, its concentration was always less

than half the concentration of Fe.

In Sage:3, though the stoichiometry and stability of the as-prepared iron

sulfide film is found to improve by choosing thioacetamide as sulfur source, this lead

to increase in the concentration of oxygen impurity.

Fe

so

Sri

u: c -C,) ~~~

u

rI

jI

ii

I-,

;---.

-~-.---,.---~~.-

--.-

--~T

-T

~--~---,

t-

~~,

·~T

68

48

17

81.

6854

4H

ind

ing

En

or

gy

(cV

)

~"ig.(5.1"/)

XI>

Sd

ep

l.h

Ill'

oIi

le(I

fl.h

inI'i

lms

am

ple

I'ro

msL

ag

e::

i

1090

80

708

05C

l40

30o

~3p

S2p

01

5.s~ Si2

p

201D

S

I i "~

./'

-_--

..r.

.........

/<

,V~"-J""'".-/

_/'

/.....

..-....

...~-'

./"--

-.....

•---

V'v~

'.r'~"-~---

_r-"

,-.",

--....

. ~'V

~~~

..:»:

__~-.-.J'..J-..---_..--'V~

Si

o'zr

?"e'

::::

><

,~------....,.,,,r>

d"'C

"":'<

'>C

>te

=M,~7?7t"-F

'!>

OO

1>

'z2

t--:7

2C

S"

c=:r-~

P8l~

A:d

<>

a

o

00 110

80 70 80» f"

"+ 0 El50

o· o 0 ::140

o ~ ::1 f""+~ ~

30f"

"+ ..... 0 ::1

~O

Spu

tter

ing

time

(min

)

Fig.

(5.1

8)P

erce

ntag

eco

mpo

siti

onfr

omX

PSan

alys

iso

fsam

ple

from

stag

e:3

5.5.4 Stage: 4

In this stage, trial was done to improve the stoichiometry of the pyrite by

applying a small potential to the substrate at the time of deposition. Sample were

prepared from solution bath containing ferric chloride and thiourea as explained in

Stage:2. Sn02 coated glass was used as substrate, in order to apply a potential at the

time of deposition. For the sake of comparison, samples were prepared with -1.5V,

+1.5V and OV applied to the substrate. Thin film prepared under +1.5V had a pale

yellow colour, while sample under

-1.5V had a deep brown colour. These samples were analysed using Secondary Ion

Mass Spectroscopy (SIMS). Fig.(5.19) shows the comparative results of these

samples. One general point noted in this analysis was the significant diffusion of

Sn02 in to the thin film surface. On comparing the figures in Fig.(5.19) it can be seen

that the intensity or counts/sec of the sulfur increased from 2xl03 to 5.5x103 on

application of a positive potential, while a negative potential reduced the counts to

1x 103• The influence of potential on the iron concentration was found to be less and it

remained in the range of 1 x 105 c/s

Hence in Stage:4 it was concluded that, though a positive potential could

influence the sulfur concentration, it was not appreciable enough to be considered for

preparation of stoichiometric iron sulfide.

5.5.5 Stage: 5

This stage involves the sulfurisation of iron oxide films from Stage:2.

Sulfurisation of thin films is a very frequently used technique adopted for attaining

the required stoichiometry of sulfides [48-51]. In the case of iron sulfide, it is already

established that the path way to FeS 2 by the action of sulfur vapour on Fe304 or Fe203

is preferred to the path Fe-» FeS ~ FeS2 [44]. The Gibbs phase triangle of Fe-O-S

system in Fig.(5.20), demonstrate equilibrium among oxide, sulfide and sulfate phase.

When sulfur acts on any iron oxide, reaction similar to the following may happen.

2 Fe202

(s) + 11 S~ 4 FeS2

(s) + 3S02(g)

Fe304

(s) + 8 S~ 3FeS2

(s) + 3S02(g)

With these information in mind, a sulfur annealing chamber was fabricated,

Fig.(5.21). It consists of two horizontal tubular sections, one the sample chamber and

the other the sulfur powder chamber. Resistive heating could be given to both the

10 6•i'

,,

,,

,i

',

'L

10 5~~/

r10 5-:f~O

104~/

/

®""

4

310

~10

\..SI~

Fe~

10J

210

10ieoo

41yn

'iJ'2

00

)=

010

\..!:!..)

Sput.Tim

e:[s]

~10 2

1~

~103

~r

Sp<A:.r::[s]

BD

ioco

106""

i,

,,

,'

,,

,'

""i""

i,

',

'L

10 2I

,,

,i

ii

II

200400

6CX

)fIX

)

Sput.Tim

e:Cs]

•(XX

)

Fig(5.19)

SIMS

spectra(a)

with

-1.5

V(b)

with

+1.5V

(c)w

ithav

___ _Thennocouple

SamplechamberSample t

, I

Inlet for H2S vapourI,,•

Source(Sulfur) chamber

vacuum pump

Fig (5.21) Schematic representation of su1furannealing chamber

5

Fe

pyriteFeS2

FeS

o

Fig (5.20) Gibbs phase triangle of Fe-O-S system

sections separately. The sample chamber could be heated to 300°C while the sulfur

chamber to 200°C.

Sample prepared in Stage:2 was the precursor for Stage:5. Sulfur powder was

introduced into the sulfur chamber and heated prior to heating of the sample chamber.

This helped to reduce the partial pressure of oxygen in the sample chamber. Initially,

samples were placed in the sample chamber in such a way that hot sulfur vapour from

the sulfur chamber could directly hit the sample surface(as shown in figure). But this

arrangement was found to damage the film, when the duration of annealing was

extended to few hours. Hence the samples were kept at the intermediate space in the

sample chamber, where direct hit of hot sulfur vapour could be avoided. The sample

chamber was heated in a slow manner and a thermocouple kept in contact with the

sample was used to sense the temperature. Annealing was done at various

temperatures like 110°C, 150°C, 190°C, 200°C, 220°C, 230°C and 270°C. Duration of

annealing was initially fixed at lhr. The effect of this sulfur annealing process was

monitored by optical absorption studies. From the initial studies it was decided to

extend the duration of annealing to 2 hr, in order to make the effect of annealing more

pronounced. It was observed that the direct band gap slowly shifted from the as­

prepared value of2.94 eV to 2.67 eV when annealed in sulfur atmosphere for 2 hrs at

200°C. But further increase in temperature lead to an increase in the band gap.

Fig.(5.22) shows a few of the selected graphs. Similarly, the indirect band gap shifted

from the as prepared value of 1.67 eV to 0.97 eV when annealed at 200°C for 2 hrs.

As in the above case, further increase in annealing temperature in found to have

reverse effect on indirect band gap. Figure (5.23) shows the (othv)" vs hv graph and

the figurative results are given as the inset. From this study it was inferred that

annealing at a temperature of 200°C lead to the conversion of iron oxide to iron

sulfide. But annealing above 200°C in the presence of air leads to the formation of

iron oxide once again. Figure (5.24) shows the scanning electron micrograph of a

sulfur-annealed film.

The XPS depth profile of such a sample annealed at 200°C for 2 hr is shown in

Fig.(5.25). The presence of Fe and S signals is obvious throughout the sample. The

binding energies around 709.6 eV and 161.9 eV for the Fe and S peaks respectively,

is found to be near the expected value of 708.6 eV and 161.7 eV [47] for pure FeS2"

100

IJ

I

4.0

3.5

3.0

1.0

....

...

unan

eale

d

-0

-an

neal

edat

150°

C

-.-

anne

aled

at19

0°C

----

anne

aled

200°

C

-+

-an

neal

ed23

0°C

--

anne

aled

270°

C

0.5

1.5

2.0

2.5

hv(e

V)

Fig.

(5.2

3)V

aria

tion

in(u

thV

)1/2

vs.

hvpl

otas

a

resu

lto

fann

eali

ngin

sulf

urat

mos

pher

e

3.5

,I

0.0

Ii

ii

ii

ii

I0.

0

3.0

2.5

1.0

0.5

El .§

2.0

~ roS ,-.

.....

~1.

5

E '-"

3.5

)lE

3.0

2.0

2.5

hv(e

V)

1.5

....

...

unan

eale

d

-0

-an

neal

edat

150°

C

-.-

anne

aled

at19

0°C

---

anne

aled

200°

C

-+

-an

neal

ed23

0°C

--

anne

aled

270°

C

O~_.-?JI_-~

I.0

.-

.-...

-...a

I

1030 20408090 70 Fig.

(5.2

2)V

aria

tion

in(a

thv)

2vs

.Il

Vpl

otas

are

sult

of

anne

alin

gin

sulf

urat

mos

pher

e.

~ .§60

~ roN

>'5

0

E '-"

Fig (5.24) SEM micrograph of sample annealed at 200°C

in sulfur atmosphere

FeS

o

50

CY

CL

ES

5

740

700

178

158

544

526

114

BIN

DIN

GE

NE

RG

Y(e

V)

Fig.

(5.2

5)X

PS

dept

hpr

ofil

eo

fsam

ple

anne

aled

at20

00

Cin

sulf

urat

mos

pher

e

96

133

But here also presence of oxygen was not negligible. This leads to the conclusion that

conversion of iron oxide to iron sulfide was not complete.

5.6 Conclusion

Iron pyrite with its high absorption coefficient, suitable band gap and eco­

friendly nature is a suitable material for photovoltaic applications. In the present work

efforts were made for the first time to prepare this material by CBD technique in

aqueous medium. Analysis of freshly prepared samples showed the sulfur

concentration in the as-prepared films to be considerably less than concentration of

iron. Moreover, it was observed that these sulfur deficient samples were unstable in

ambient conditions and it slowly gets converted to iron oxide. Hence it was concluded

that it is impossible to deposit iron sulfide by using CBD technique. By the process of

sulfurisation, the sulfur content could be increased to some extent. However, as

annealing was tried in the presence of oxygen the formation of iron oxide could not

be avoided at higher temperatures. The possible suggestion for improvement of

stoichiometry is to anneal the sample for longer durations and at higher temperatures

in the absence of air. This may lead to the replacement of oxygen by sulfur, thuse

resulting in pyrite.

Reference

[1] A. Ennaoui and H. Tributsh, Solar Cells, 13 (1984) 197.

[2] T. A. Bither, R. J Bouchard, W. H. Cloud, P. C. Donohue and W. J. Siemons,

Inorg. Chem., 7 (1968) 2208.

[3] A. Ennaoui, S. Fiechter, H. Goslowsky and H. Tributsch, J. Electrochem. Soc.,

132 (1985) 1580.

[4] S. Bausch, B. Sailer, H. Keppner, G. Willeke, E. Bucher and G. Frommeyer,

Appl. Phys. Lett., 57 (1990) 25.

[5] I. J. Ferrer, D. M. Nevskaia, C. de las Heras and C. Sanchez, Solid State

Commun., 74 (1990) 913.

[6] A. Ennaoui, S. Fiechter, Ch. Pettenkofer, N. Alonso-Vante, K. Buker, M.

Bronold, Ch. Hoptner and H. Tributsch, Sol. Energy Mater. Sol. Cells, 29 (1993)

289.

[7] A. Ennaoui, S. Fiechter and H. Tributsch, M. Giersig, R. Vogel and H. Weller, J.

Electrochem. Soc., 139 (1992) 2514.

[8] W. W. Kou and M. S. Seehra, Phys. Rev. B, 18 (1978) 7062.

[9] C. de las Heras, I. J. Ferrer and C. Sanchez, J. Appl. Phys, 74 (1993) 4551.

[10] J. Pankove, Optical Process in Semiconductors, Ch. 3 (1971) 34.

[11] F. Basani and G. Pastoriu, Electronic States and Optical Transmissions in solids,

Ch.5 (1975) 149.

[12] K. Seegar, Semiconductor Physics- An Introduction, Ch.l1 (1985) 302.

[13] D. W. Bullett, J. Phys. C: Solid State Phys., 15 (1982) 6163.

[14] A. L. Echarri and C. Sanchez, Solid State Commu., 15 (1974) 827.

[15] R. Schieck, A. Hartmann, S. Fiechter, R. Konenkamp and H. Wetzel, J. Mater.

Res.,5 (1990) 1567.

[16] J. R. Ares, M. Leon, N. M. Arozamena, J. Sanchez- Paramo, P. Celis, I. J. Ferrer

and C. Sanchez, J. Phys. Condense Mater.", 10 (1998) 4281.

[17] B. Rezig, H. Dahman and M. Kenzari, Renewable Energy, 2 (1992) 125.

[18] G. Willeke, R. Dasbach, B. Sailer and E. Bucher, Thin Solid Films, 213 (1992)

271.

[19] O. Blenk, E. Bucher and G. Willeke, Appl. Phys. Lett., 62 (1993) 2093.

135

[20] A. Emaoui, S. Fiechter, W. Jargermann and H. Tributsh, J. Electrochem. Soc.,

133 (1986) 97.

[21] A. Ennaoui and H. Tributsch, Sol. Energy Mater., 14 (1986) 461.

[22] A. Ennaoui and H. Tributsch, 1. Electroannal. Chem., 204 (1986) 185.

[23] K. Buker, N. Alonso - Vante and H. Tributsch, J. Appl. Phys., 72 (1992) 5721.

[24] N. Bronold, C. Pettenkofer and W. Jargennann, 1. Appl. Phys, 76 (1994) 5800.

[25] J. R. Ares, I. J. Ferrer, F. Cuevas and C. R Sanches, Thin Solid Films, 387(2001)

97.

[26] C. de las Heras, J. L. Martin de Vidales, I. J. Ferrer and C. R Sanches, J. Meter.

Res., 11 (1995) 211.

[27] G. Pimenta, W. Sehroder and W. Kantek Ber. Bunsenges, Phys-Chem., 95 (1991)

1470.

[28] G. Pimenta and W. Kautek, Thin Solid Films, 219 (1992) 37.

[29] S. Bausch, B. Sailer, H. Keppner, G. Willeke and E. Bucher, Appl. Phy. Lett., 57

(1990) 25.

[30] I. J. Ferrer and C. Sanchez,1. Appl. Phys., 70 (1991) 2641.

[31] D. Lichtenberger, K. Ellmer, R. Schieck and S. Fiechter, Appl. Surface Sci., 70

(1993) 583.

[32] M. Birkholz, D. Lichtenberger, C. Hopfner and S. Fiechter, Sol. Energy Mater.

Sol. Cells, 27 (1992) 243.

[33] A. Ennaoui, G. Schlichthorl, S. Fiechter and H. Tributsh, Sol. Energy Mater. Sol.

Cells, 25 (1992) 169.

[34] C. de las Heras and C. Sanchez, Thin Solid Films, 199 (1991) 259.

[35] G. Chatzitheodorus, S. Fiechter, R. Konenkamp, M. Kunst, W. Jaegermann and

H. Tributsch, Met. Res. Bull., 21(1986)1481.

[36] B. Thomas, K. Ellmer, M. Muller, C. Hopfner, S. Fiechter and H. Tributsch, J.

Crystal Growth, 170 (1997) 808.

[37] A. Ennaoui, S. Schroetter, S. Fiechter and H. Tributsch, J. Mater. Sci. Lett., 11

(1992) 1131.

[38] Sigeyuki Nakamura and Akio Yamamoto, Sol.Energy Materials & Sol. Cells, 65

(2001) 79.

[39] S. Fiechter, J. Mai and A. Ennaoui, J. Crystal Growth, 78 (1986) 438.

[40] G. Smestad, Sol. Energy Mat., 18 (1989) 299.

136

[41] A. K. Abass, Z. A. Ahmed and R. E. Tahir, Phys. State Sol. (a), 97 (1986) 243.

[42] A. K. Abass, Z. A. Ahmed and R. E. Tahir, J. Appl. Phys., 61(1987) 2339.

[43] Chatzitheodorou, S. Fiechter, M. Kunst, J. Luck and H. Tributsch, Mat. Res.

Bull., 23 (1988) 1261.

[44] G. Smestad, A. Ennaoui, S. Fiechter, H. Tributsch, W. K. Hofmann and M.

Birkholz, Sol. Energy. Mat., 20 (1990) 149.

[45] Liu Chongyang, C. Pettenkofer and H .Tributsch, Surface Sci., 204 (1988) 537.

[46] A. N. Buckley and R. Woods, Appl. Surface Sci., 27 (1987) 437.

[47] G. Panzner and B. Egert, Surface Sci., 144 (1984) 651.

[48] T. Ohashi, K. Inakoshi, Y. Hashimoto and K. Ito, Sol. Energy Mater. Sol. Cells,

50 (1998) 37.

[49] R. P Wijesundara, L. D. R. D. Pereram K. D. Jayasuriya, W. Siripala, K. T. L. De

Silva, A. P. Samantilleke and I. M Dharmadasa, Sol. Energy Mater. Sol. Cells,

61 (2000) 277.

[50] F. O. Adurodija, J. Song, S. D Kim, S. K. Kim and K. H. Yoon, Jpn. J. Appl

Phys., 37 (1988) 4248.

[51] M. Abaab, M. Kanzari, B. Rezig and M. Brunel, Sol. Energy Mater. Sol. Cells,

59 (1999) 299.